This invention relates to solenoidal electric field (SEF) lamps and in particular to a method and apparatus for dimming SEF lamps.
SEF lamps are efficient, fluorescent type lamps configured and operated in a fashion so as to serve as replacements for conventional incandescent lamps which operate at low energy efficiencies; in particular, an energy efficiency for an incandescent lamp of 15 lumens per watt is typical while the energy efficiency of SEF lamps can exceed 50 lumens/watts. Moreover, incandescent lamps exhibit a relatively short life time even when compared to conventional tubular fluorescent lamps. However, SEF lamps are efficient and long lived. Such SEF lamps are described, for example, in U.S. Pat. No. 4,128,785 issued to Ferro et al. and in U.S. Pat. No. 4,117,378 issued to Glascock, both of which are incorporated herein as background material. While these SEF lamps are efficient and long lived, up until now there has been no effective way to dim these lamps without deleteriously affecting the lamp efficiency. Since lamp dimming is both an aesthetically desirable feature and since it can also result in energy saving through reduced power consumption, it is very desirable that SEF lamps operate in this manner.
All SEF lamps contain an ionizable medium through which an electrical current flows. This ionizable medium exhibits a negative resistance characteristic, that is to say, as the discharge current through the plasma increases, the voltage drop along the discharge path increases. Thus, lowering the peak currents in SEF lamps results in an increased operating voltage which causes a corresponding increase, in a near cubic fashion in the flux density in the toroidal core of the lamp. This results in excessive core losses, reduced efficiency, and it increases the possibility that the Curie temperature of the core is exceeded. Likewise, dimming based on the use of full or half-wave rectified sinusoidal currents is not efficacious since SEF lamps will not turn on until well into the current cycle, particularly for lamps operating at a high power factor. When such lamps are running at high power factor, lamp voltage is changing from peak value to zero during the "on" portion of the lamp cycle and thus sinusoidal frequency is decreasing through each of these same peak currents. Such a lamp inherently turns off at some arbitrary time when the applied voltage is less than the reflected lamp voltage. To achieve a given average power level, instantaneous or cycle-to-cycle power levels must be considerably higher during early portions of the cycle. Since lamp and ballast inefficiencies result, it is desirable to run at a constant power level which means low power factor operation. Thus, in low power factor lamp operation, the only method of reducing power to the lamp is to reduce the peak or RMS current levels; however, as seen above, this results in core heating and inefficiency, and additionally, results in plasma saturation which results in an approximately 12 percent decrease in efficiency. Also, dimming in SEF lamps operating at high power factor with sinusoidal waveforms results in lack of full lamp control and lowered efficiency. While low power factor operation is preferred to high power factor operation, dimming in a low power factor mode is inefficient with conventional dimming methods, and accordingly, the present invention alleviates this problem.